This invention relates to an electro-optical device and more particularly to an image intensifier tube and an optical arrangement for such a tube.
Image intensifier devices multiply the amount of incident light they receive and thus provide an increase in light output which can be supplied either to a camera or directly to the eyes of a viewer. These devices are particularly useful for providing images from dark regions and have both industrial and military application. For example, these devices are used for enhancing the night vision of aviators, for photographing astronomical bodies and for providing night vision to sufferers of retinitis pigmentosa (night blindness).
Modern image intensifier devices include three main components, namely a photocathode, a phosphor screen (anode) and a microchannel plate (MCP) positioned intermediate to the photocathode and anode. These components are housed in a tube. The photocathode is extremely sensitive to low-radiation levels of infrared light in the 580-900 nm (red) spectral range. The MCP is a thin glass plate having an array of microscopic holes through it. Each hole is capable of acting as a channel-type secondary emission electron multiplier. When the microchannel plate is placed in the plane of an electron image in an intensifier tube, one can achieve a gain of up to several thousand. Since each channel in a micro-channel plate operates nearly independently of all the others, a bright point source of light will saturate a few channels but will not spread out over adjacent areas. This characteristic of "local saturation" makes these tubes more immune to blooming at bright areas.
The anode of the image intensifier tube includes an output window and a phosphor screen which is formed on one surface of the window. Known tubes have included the use of a flat glass window as an output screen for the image intensifier. However, the two parallel glass surfaces of the window cause reflections and ghost images which cannot be eliminated by the use of anti-reflective coatings. In order to overcome the reflection problem, a fiber optic output element is normally used instead of a flat glass output window.
The fiber optic window is comprised of a matrix of very thin core glass rods surrounded by a clad glass. It is a high cost component. In addition, approximately 35% of the surface area of the fiber optic window is blocked from receiving light due to the matrix construction. Thus, the performance of the device is degraded due mainly to the relatively low open or optically usable area of the fiber optic window, which is typically about 60%.
It is therefore an object of the present invention to provide an image intensifier device having a low reflection optical window.
It is an additional object of the present invention to provide a method of making such an output window in a highly economical and efficient manner.
These objects and others which will become apparent hereinafter are accomplished by the present invention which provides an image intensifier device including an input window formed of optical material and having light receiving and light transmitting surfaces; photoemissive means on the light transmitting surface for emitting electrons in response to light received at the photoemissive means; means positioned adjacent the photoemissive means for amplifying the number of electrons emitted from the photoemissive means; means positioned adjacent the amplifying means for converting the energy from the amplified electrons to light; and means for focusing the image received from the converting means.
The present invention also provides a method of making an image intensifier device including the steps of forming a cathode for receiving input light and emitting electrons in response to the received light; providing means for amplifying the number of electrons emitted from the cathode; positioning an anode adjacent the amplifying means for converting the amplified electrons to light rays; and placing an optical element adjacent the anode for converting the light rays to an image.